Technical Abstract:
Atrazine and metolachlor are heavily applied to agricultural fields in the US, and their respective metabolites are often found in greater quantities in surface and ground waters, as well as soils, than the corresponding parent compounds. Because the amount of chemical which can potentially reach ground and surface waters is indicative of that which can be extracted by soil water, the water extractable phase of a compound should reasonably estimate the amount of chemical available for transport. Quantification of water extractable metolachlor, atrazine and a suite of their respective metabolites in surface soils has been carried out on a sub-watershed at the Beltsville Agricultural Research Station. Soils were collected on a regularly spaced geo-referenced grid so that patterns of dissipation relating to soil-specific characteristics could be identified. Results indicate an initial rapid decrease in both parent compounds within 6 d after application (metolachlor falling from 22 ppm to 4.4 ppm, and atrazine declining from 25 ppm to 5.7 ppm). A simultaneous gradual increase in two metabolites of metolachlor, metolachlor ethane sulfonic acid, and metolachlor oxanilic acid, occurs until 6 d after application and then increases significantly by the 15th day. Three common atrazine metabolites, OIET, CEAT, and CIAT, also increase until 6 d after application, but then decrease over the next 15 d after application. Metabolites of both metolachlor and atrazine are present even on the day of application evidencing the rapid degradation of both compounds. Differences in the dissipation patterns for both compounds at the individual sample locations were also observed.